Just bought a '78 GS750. Ran great for the most part after I took it for a good little drive, aside from a sporadic miss on cylinder #4 (? the far right one when sitting on the bike). Sometimes, more rarely, #1 would go dead, and I have a feeling that it may just be a loose wire at the spark plug....haven't fully concluded this, though. So two sporadic cylinders made me check the compression first thing, just to eliminate things since I suspected the carbs.
Well the results and plug colors are below:
#1: ~90psi, a good healthy tan rich color
#2: ~90psi, no color, almost looked new.....not lean, though!?
#3: ~30-40psi, no color like above
#4: ~30-40psi, a bit rich, a good bit of black carbon and some oily soot
All plugs looked like they were fairly brand new, indicating the previous owner was trying to work through the problems himself. I let it cool down, then cranked it and touched the pipes. #1 got hot like normal, #2 just a HAIR slower, #3 got hot but very slowly, and #4 didn't hardly get hot at all. Compression test with a cap full of oil brought the compression up from 30ish psi to 65ish psi.
#1's occasional misfire is not explained, but apparently I've got two near-dead cylinders, even though #3 still fires strong. Pressure came up with oil added, but still not enough so I'd say it indicates bad valves? No visible leaks for a head gasket or anything. Anyone got any advice? I don't have a manual so I don't know what are acceptable compression numbers on this motor, I was just comparing between cylinders. I gotta try another compression tester tomorrow morning as well, to make sure my gauge isn't reading a bit low.
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